The present invention relates generally to high efficiency filters and filtering processes and more particularly to filtering of fine particles from fluid streams. It relates also to the form of the high efficiency filters and to the manner of their manufacture.
The design of filters to remove particulate material from gases is known to depend on the temperature and compositions of the gases containing the particles and on the size and character of the particles to be removed.
The removal of fine particles of coal, carbon and ash from hot gas entering a gas turbine through use of ceramic filters to prevent damage to turbine blades is taught in U S. Pat. No. 4,343,631. Numerous other patents teach specific filter structures useful in separating designated materials from specified fluid streams. Such patents include U.S. Pat. Nos. 3,880,620; 4,530,418; 4,734,195; 3,707,831; 3,031,827; 3,766,629; 4,363,644; 4,689,059; 3,491,891; 4,640,779; 3,502,596; 2,076,304; 3,330,101; 4,613,438; 3,883,331; 2,885,026; 3,594,989; 4,639,261; 4,584,005; 4,685,944; 4,233,044 and 2,980,208. A number of these patents disclose a filter structure involving a honeycomb like array of filter elements or filter channels. However in essentially all such filter elements the design of the filter is for a fluid flow parallel to the axis of the honeycomb. The subject application deals with a filtering structure and filtering operation in which the filtering occurs in a direction perpendicular to the axis of the cells of a cellular structure such as a honeycomb.
A set of standards has been established for the operation of filters in removing finer and finer particles from gas. This standard has been set with respect to clearing the air for so-called clean room operations. The standard is known as Federal Standard 209B and it is entitled "Clean Room and Clean Work Station Requirements, Controlled Environment". The standard was established partly in connection with clean room operations used in forming microelectronic chips because of the negative impact of air borne dust particles on the manufacture and on the operation of such chips. The standard contains a number of criteria. Some of these criteria have to do with the number and size of particles which must be removed from air and conversely the number and size of particles which may remain in the air of a "clean room." Recently there have been a number of studies and proposals relating to the modification of the standard and a new standard, FED-STD-209D dated June 1988 has been published. A copy of the standard, amounting to some 41 Pages, was published in the Journal of Environmental Science of Sept/Oct 1988.
Some of the filters which are used in connection with establishing a clean room atmosphere are known as HEPA (or High Efficiency Particulate Air) filters. There are carefully drawn criteria about how such filters are to be operated and the ability which they must display for removing particles of prescribed sizes from air. One of these criteria is that the resistance to airflow of the filters at prescribed airflow should not be more than 1.0 inch of water except for filters which have an efficiency of 99.999% and for these latter filters the pressure drop is established by agreement of buyer and seller.
A HEPA filter is defined by the standard as a throw-away extended-media dry-type filter in a rigid frame having minimum particle collection efficiency of 99.97% for 0.3 micrometer thermally-generated dioctyl phthalate (DOP) particles, and a maximum clean-filter pressure drop of 1.0 in. water gage, when tested at rated air-flow capacity.
Another type of filter is known as an ASRAE filter and still another is known as a UPLA (Ultra Low Particle Air) filter. Another filter is known as the ULPA filter (or Ultralow Penetration Air Filter). Each of these filters has well defined and well recognized characteristics which are well known in the trade.
One characteristic which is shared by all of these filters is that each such filter has just one filter media layer. The subject invention concerns a new and unique multilayer filter media system for the above applications which has two or more layers of filter media.